1. Field of the Invention
The present invention relates to a tensioner for applying a given tension to a force transmitting member such as a camshaft driving chain or timing belt of an engine mounted in a vehicle, e.g., a four-wheeler, two-wheeler, etc.
2. Description of the Related Art
A tensioner is used to keep a chain and timing belt under a substantially fixed tension even if they are stretched or worn to be slackened during use. A conventional prevalent tensioner comprises a casing, a first shaft member having an external thread portion, a second shaft member having an internal thread portion in mesh with the external thread portion of the first shaft, a spring for urging the first shaft member in a first rotating direction, a bearing for restraining the second shaft member from rotating, etc. When the first shaft member is rotated in the first direction by means of the aforesaid spring, the second shaft member moves in the axial direction. The first shaft member is stored in the casing, and an end face of the first shaft member is in contact with the inner surface (receiving surface) of the casing.
This tensioner urges the first shaft member in the first rotating direction by means of a repulsive force (elastic energy) the aforesaid spring accumulates when it is twisted in the direction opposite to the first rotating direction. Urged by its own turning torque, the second shaft member moves in the direction to project from the casing. The distal end of the second shaft member directly or indirectly pushes a force transmitting member such as a chain or timing belt.
If the tension of the chain or timing belt increases as an engine rotates at high speed, for example, a force that pushes back the second shaft member is augmented. In this case, the second shaft member is pushed back into the casing, resisting the sum total of torques, mainly including the urging force of the aforesaid spring, frictional resistance between the aforesaid external and internal thread portions, and frictional resistance between the end face of the first shaft member and the casing. Based on these torques, the tensioner can apply a fixed tension to the chain or timing belt.
The second shaft member of the tensioner described above is expected to be able to advance and retreat smoothly depending on the tension of the chain or timing belt. In the conventional tensioner, however, frictional torque between the end face of the first shaft member and the receiving surface of the casing sometimes vary substantially. Thus, there is a problem that the second shaft member cannot advance and retreat smoothly. When the engine is in a low-speed rotation zone for idling operation or the like, in particular, the return characteristic of the second shaft member may be poor, or the initial characteristics of the tensioner may be unstable if the tensioner is a brand-new one having just started to be used. Accordingly, measures are taken to optimize the lead angle of each of the aforesaid thread portions and adjust the spring constant of the aforesaid spring. Despite these measures, however, there still remain problems that the initial characteristics of the tensioner are subject to substantial fluctuations and that the change of the frictional resistance is great.
Accordingly, the object the of the prevent invention is to provide a tensioner designed so that its initial characteristics can be stabilized, change in its characteristics during operation can be reduced, and optimum specifications can be set corresponding to an engine in which it is attached.
In order to achieve the above object, a tensioner of the present invention comprises a casing, a first shaft member rotatably stored in the casing so as to be restrained from moving along an axis, a second shaft member in screwed engagement with the first shaft member, movable in the direction of the axis, restrained from rotating relatively to the casing, and subjected to a load in the direction of the axis from a force transmitting member, and a spring stored in the casing and capable of applying turning force to the first shaft member, characterized in that an end face of the first shaft member touches a receiving surface of the casing directly or across a washer and receives the load acting on the second shaft member, the flatness of at least the end face of the first shaft member being 15 xcexcm or less.
In the tensioner of this invention, the axial load from the force transmitting member, e.g., a chain or timing belt, applied to the tensioner is supported by the receiving surface of the casing through the medium of the end face of the first shaft member. Since the flatness of the end face of the first shaft member is 15 xcexcm or less, the first shaft member can rotate smoothly. As the first shaft member rotates, the second shaft member smoothly moves in the axial direction with respect to the casing, so that the return characteristic of the second shaft member is improved.
In the present invention, the end face of the first shaft member may be formed having a chamfer portion on the outer peripheral edge thereof, the flatness of the whole end face except the chamfer portion being 15 xcexcm or less. As the first shaft member rotates around the axis with respect to the receiving surface of the casing, in this invention, the whole end face smoothly slides with respect to the casing, so that a stable return characteristic can be obtained.
In the present invention, the flatness may be adjusted so that the flatness of an annular portion of the end face extending in the circumferential direction thereof is 15 xcexcm or less. As the first shaft member rotates around the axis with respect to the receiving surface of the casing, in this invention, the annular portion of the end face extending in the circumferential direction smoothly slides, so that a satisfactory return characteristic can be enjoyed.
Preferably, in the present invention, the surface roughness of the end face of the first shaft member should be 5 xcexcm or less. According to this invention, the first shaft member can rotate more smoothly, so that a satisfactory return characteristic can be enjoyed.